The present invention provides an in-line process for the direct production of high-temperature resistant glass fibers. In one embodiment, this new in-line process includes a top-charging electric glass-melting furnace positioned vertically above a bushing plate containing apertures through which fibers are extruded, and an intermediate flow regulating baffle system which improves the heat pattern across the glass flow by redirecting the glass flow to the bushing. In addition, a bushing of enlarged size and heretofore unknown design allows drawing a significantly increased number of glass fibers at one time at high temperatures while minimizing bushing deformation or sag. The invention further relates to a high-strength/high-temperature magnesium aluminosilicate fiber designed for use in filament winding, pultrusion and weaving processes.
The process and apparatus of the present invention are useful in a wide range of applications relating to the production of continuous high-temperature glass fibers, strands and filaments. In particular, the process and apparatus permit the direct production of fiber rovings for use as a reinforcing material in composite structures. Additionally, the glass fibers of the present invention are useful in a diverse range of applications requiring high strength and/or high-temperature stability. For example, glass fibers of the invention are useful in thermal acoustical silencers such as those used with gas engines, diesel generators and jet turbines.
Glass fibers are well known and widely used as reinforcing materials in a wide variety of composite structures. While both continuous and staple glass fibers are used as reinforcing media in such structures, continuous glass fibers are generally preferred for applications requiring high-strength materials. For example, continuous glass fibers are often used in military applications, such as in aircraft radar protection domes, rotor blades and armor systems, as well as in commercial pressure vessels and jet components. In these applications, a tow or roving comprised of a multiplicity of individual filaments or fibers are incorporated into the structure. Typically, this collection contains between 1500 and 24,000 individual glass filaments having a diameter of from about 5 microns to 30 microns, depending on the product.
The use of continuous glass fibers in high-temperature environments and to reinforce high-strength composite materials has placed increased demands on the strength and high-temperature stability of the glass fibers. As a result, researchers are constantly looking for glass compositions that provide stronger fibers capable of withstanding higher temperatures without deformation, and for ways to improve the manufacturing process to achieve greater consistency and uniformity among the fibers. Accordingly, a need exists for glass compositions exhibiting higher strength and higher temperature stability than the conventional magnesium aluminosilicate glasses typically used in such applications containing approximately 65% silica (SiO2), 25% alumina (Al2O3) and 10% magnesia (MgO).
Additionally, a need exists for manufacturing methods that maximize uniformity of the fibers and minimize fiber breakage. Because of the high melting temperature of the magnesium aluminosilicate glasses typically used for such high-strength glass reinforcing fibers, it has heretofore not been feasible to produce such rovings from a single fiber-forming apparatus such as disclosed in U.S. Pat. No. 3,264,076. As a result of the high temperatures necessary to melt such compositions, the conventional bushings through which such compositions are drawn into fibers have been limited in size to those capable of producing from approximately 200 to 1000 fibers of from about 5 microns to 14 microns in diameter. Attempts to use larger bushings with greater numbers of tips or orifices through which the glass is drawn have typically encountered the problem of the bushing sagging or warping as a result of the high temperature and the weight of the larger quantities of molten glass supported by the bushing. Such warpage of the bushing is generally unacceptable for numerous reasons. For example, since the bushings are typically heated via electrical resistance, the bushing must be electrically insulated to operate effectively. Warping of the bushing allows the lower surface of the bushing to contact the heat-dissipating fins located beneath the bushing, voiding the electrical isolation of the bushing and resulting in the occurrence of xe2x80x9ccold spotsxe2x80x9d on the bushing at such points of contact. Additionally, warping of the bushing alters the position of the bushing tips and can result in the fibers contacting one another, causing breakouts and stopping the fiberizing process. Further, warping of the bushing can cause a distortion of the orifices through which the fibers are drawn and result in irregularly shaped fibers having less than optimum properties.
As a result of the inability of conventional processes and equipment to permit the direct formation of a sufficient number of fibers to make a roving from a single bushing, it is conventional practice for the fibers exiting the bushing to be drawn from the bushing into forming strands containing from about 200 to 400 fibers. The fibers of the forming strands receive a coating of a sizing composition containing lubricants, among other things, to minimize abrasion of the fibers during handling. After sizing, the forming strands are individually wound into rolls for storage. Rovings used as reinforcing materials are then made by combining a sufficient number of forming strands to provide a roving having the desired number of fibers. In this operation, the forming strands are unwound from their individual rolls, combined to form the roving, and the roving is then wound onto a roll for shipment to the end users. Unfortunately, the winding and unwinding of the forming strands onto rolls, and the combining of the forming strands into a roving, impart stresses to the strands and introduce a significant amount of fiber breakage, which results in a roving having less than optimal strength characteristics.
Moreover, combining a number of forming strands to make the roving typically results in a roving having undesirable catenary when the roving is unwound for use. Accordingly, a need exists for a fiber-forming apparatus capable of operating at the high temperatures necessary to melt high-strength, high-temperature magnesium aluminosilicate glasses, and of directly forming the number of filaments desired in the final roving. As such, a need exists for a bushing design that is large enough to accommodate the quantity of tips or orifices required to create the desired number of fibers, yet withstand the heat and weight of the molten glass load without warpage or sagging.
These heretofore unmet needs are satisfied by the glass and sizing compositions, and the fiber-forming process and apparatus, of the present invention.
The present invention provides an in-line process for the direct production of glass fiber rovings formed of more than about 850 substantially continuous high-strength, high-temperature glass fibers comprising: (1) heating a high-temperature glass composition to a temperature sufficient to form a melt; (2) passing the melt through a fiber-forming bushing having a tip plate assembly formed of one or more plates, the tip plate assembly containing a sufficient number of fiber-forming tips to simultaneously form more than about 850 substantially continuous glass fibers; (3) applying a sizing composition to the fibers; and (4) collecting the substantially continuous glass fibers directly into a roving. This in-line process allows for the production of magnesium aluminosilicate glass fibers having improved strength, high-temperature stability, and corrosion resistance. In addition, the in-line process of the present invention reduces devitrification at high temperature, thus reducing fiber variability. The process additionally provides for better fiber alignment in a single-step roving-forming operation, resulting in a reduction in catenary and broken filaments and improved translation of fiber properties to fiber-reinforced composite structures.
In one embodiment of this innovative process, the fibers are formed by melting the glass composition in an electric furnace located vertically above a bushing tip plate assembly through which the fibers are extruded. The bushing tip plate assembly of the invention is designed to reduce tip plate sag at high temperatures while permitting a greater number of fibers to be drawn therefrom than heretofore possible. In another embodiment of the present invention, a flow regulation system is included in the glass flow channel between the furnace and bushing tip plate assembly to provide a more uniform glass temperature distribution across the bushing tip plate assembly, improve the bushing operating efficiency, and allow for a tighter control of filament diameter.
In a preferred embodiment, the bushing for making continuous glass fibers comprises a generally rectangular, elongated tip plate assembly including one or more plates each having a plurality of holes therein, walls extending upwardly from the plate along all sides thereof forming a cavity, ribs projecting into the cavity formed by the walls from two oppositely faced walls in an alternating pattern and projecting substantially perpendicular to the adjoining walls, the ribs being attached to the adjoining wall and to each plate, the bushing being capable of continuous use at temperatures of from 2850xc2x0 F. to 2950xc2x0 F. for periods of from 18 to 21 days without experiencing sag in the tip plate assembly greater than about 0.094 inches. The tips are preferably attached to the tip plate at the holes thereof, the tips each having a bore therethrough through which glass passes. The tips may be removably inserted, whereby tips having different diameter bores may be inserted. Preferably, the plurality of holes are from 1,500 to 2000 in number. Each bore preferably has a diameter of from about 1200 microns to about 1500 microns, more preferably of from about 1300 microns to about 1400 microns. Preferably, the holes are distributed uniformly through the longitudinal area of the tip plate assembly, and are aligned in rows traversing the width of the tip plate assembly. The holes may further be arranged in double rows between adjacent ribs, with the holes in each of the double rows being out of alignment with the lengthwise axis of the tip plate assembly. The tip plate assembly preferably has a width of from about 2.5 inches to about 4.5 inches, and a length of from about 17 inches to about 21 inches. The ribs preferably span from 50 to 100 percent of the width of said tip plate assembly, with each rib having a width that diminishes along the length of the rib away from the adjacent wall to which it is attached. Preferably, the ribs span the entire width of the tip plate assembly and are attached to the assembly and both of the oppositely facing walls. Preferably, each rib has a width that diminishes to a minimum along the length of the rib away from one of the oppositely facing walls to which it is attached and therefrom increases along the length of the rib to the other opposing wall to which it is attached.
In addition, the process of the invention further includes the coating of the glass fibers with a sizing composition not heretofore used in the manufacture of high-strength, high-temperature glass fibers. This composition preferably comprises a dual-silane system designed to increase the thermal hydrolytic stability of the fibers by reducing the diffusion of moisture and ionic activity leading to corrosion.
The above and other objects and advantages of the invention will become apparent from the detailed description below in conjunction with the appended drawing figures.